What is the best weight configuration to do aero in the 4 ? With 1 or 2 pelople?
You'll never meet the aerobatic gross weight limits with 2 people aboard.
The acro gross weight of the -4 is 1375 lbs, so if you have 2 people in it, you will most likely be over that weight. Also, with an aft CG, it is easy to pull too many G's because the stick forces are very light. With just pilot, it is very balanced
(assuming O-320 and lightweight prop).
BTW, I am a IAH based, Jetlink pilot. Are you (based on your user name)? PM me
I have understood it that the max weight doing aerobatics is calculated by extracting the weight of the fuel. The fuel tanks are lifting bodies (the wings), so there will be no added stress on the main spar with more fuel.
Not true. Fuel in the RV airplanes is carried inboard on the wing. The fuel weight absolutely DOES influence wing spar shear and bending loads. Only fuel carried right on the tip is "flying" weight, and then you have the issue of secondary loads on the tips.
Fuel weight MUST be factored into your overall gross and applied against the limits Van publishes for operation of the airplane.
-Bill
Subject: Re: For max aerobatic gross weight, does fuel count?
From: "Dick VanGrunsven"
Date: Tue, 2 May 2006 11:32:40 -0700
To: Kevin Hester
Kevin,
The aerobatic gross weight of the RVs (with wing tanks) is
basically the zero fuel wt. Weight of Fuel in the wings has little
effect on the spar bending loads caused by G-forces. That said, it
is still a good idea to keep fuel to a minimum during aerobatics.
More fuel in the wings affects rolling inertia and increases stall
speed. The lightest airplane is the most aerobatic.
Dick VanGrunsven
The aerobatic gross weight of the RVs (with wing tanks) is
basically the zero fuel wt. Weight of Fuel in the wings has little
effect on the spar bending loads caused by G-forces.
With all respect, I have to disagree on this one. There is no problem designing the aircraft so that the statement "The aerobatic gross weight of the RVs (with wing tanks) is basically the zero fuel wt" is true. Besides, wings are flying weight as per def, they just happen to be built much stronger than needed to carry only their own weight.The above quote posted in a previous reply within this thread is said to be from Dick himself.
Unfortunately, who ever stated this is dead wrong. Any competent aero engineer understands the need to detail both mass distribution as well as spanwise lift distribution in order to build the basis for calculating structural stresses. Weight carried by the wing itself is - without any question - a factor in spar stress. This business of "flying weight" is the product of misguided intuition. This is basic aero engineering knowlege.
Ask yourself the question: what certified airplane allows the pilot to deduct fuel weight when measuring compliance with structural loading issues? The answer: none. And this is simply because aero engineers know where the fuel is loaded; they run their stress models using that information and the end result is passed on to you in a properly developed flight manual.
It is true that fuel carried outboard on the wing reduces bending and shear loads - on that portion of wing that's inboard of the load. But what if we move the fuel load to mid-span? How about inboard tanks like all the RV's have? What if all the fuel is carried in a fuselage tank? You see, fuel carried inboard drives stresses higher on every inch of wing outboard of the fuel load - like your RV.
The only reason I'm expounding on this here is that it absolutely is a safety issue. More than one RV has shed wings in flight. If you insist on deducting the fuel from the "aerobatic" weight limit, go ahead and put 4g's on your bird at a relatively high weight. While you're doing it, take a look at your leading edge wing skins just outboard of the fuel cap. You just might rethink your decision.
Respectfully,
Bill Wightman
More than one RV has shed wings in flight.
Bill Wightman
With all respect, I have to disagree on this one. There is no problem designing the aircraft so that the statement "The aerobatic gross weight of the RVs (with wing tanks) is basically the zero fuel wt" is true. Besides, wings are flying weight as per def, they just happen to be built much stronger than needed to carry only their own weight.
The extra weight of the fuel has to be caried somehow, and this will increase the total loading on the wing, but extra loading is not automatically the same as bending forces on the spar. So, if the spar outside of the fuel tank is strong enough to carry the extra weight of the fuel, and the spar inside the tank is strong enough to carry the weight minus the fuel, there should be no problem.
Anyway, the point is that if the wing is designed as the ref say it is, then I don't see any reason to doubt it. On the other hand that particular design feature is not exactly official in the right sense of the word.
Pierre,
Yes the 8 spar did fail just outboard of the fuel tank. The "old" RV8 spar unfortunately was designed with three structural discontinuities all bunched fairly close together. I know - I did a very detailed forensic study of it. The three disco's (sharp changes in cross-sectional moment, or structural changes with span) were 1. Fuel tank/outboard skin juncture 2. Wing spar shear web stiffener termination 3. Wing spar cap thickness step-down.
Aero structures are elastic and must be designed to smoothly distribute moment, cross sectional area, and mechanical breaks with span. Its a bad idea to allow them to get grouped together. Do that and you get a wing that tends to "fold" at one point rather than to bend smoothly with span. Again, note how your wings show deformation under load. On my 4 it was very concentrated just outboard of the fuel cap, a sight that never sat well with me. But the airplane has proven safe over the years with many thousands of RV4 hours flown without incident. Kudos to Van on that.
Now couple the above discontinuities with the large amount of span carried outboard (ie lift generated outboard) on a rectangular planform and you get big bending loads inboard on the wing (SvingenB you listening here?) After the crash and investigation, the spar was redesigned by extending the anti-buckling plate outboard, and restricting the "old" spars to reduced gross weight. Wonder if any of us feel frisky enough to subtract the fuel weight off these airplanes when thinking about doing some acro....
Bill
In the truest sense for designing a structure like a wing with the best weight to strength ratio, you would have no discontinuities along the span. In the perfect world the structure would very in a manor that was linear to the change in load along the span. But, just because a wing is not that way, doesn't mean it is not strong enough in the area where the "discos" occur. It might be that the wing is actually carrying extra weight because it has extra structure that is not needed (but may be that way for simplification of the structure, etc.). In the case of the orig. RV-8 wing, it was proven by additional testing that it met the design requirements even with the discos.
The above quote posted in a previous reply within this thread is said to be from Dick himself.